Synergistic effect of peroxisome proliferator activated receptor-γ and liver X receptor-α in the regulation of inflammation in macrophages

Peroxisome proliferator-activated receptor-gamma (PPAR gamma) and liver X receptor-alpha (LXR alpha) are nuclear ligand-activated transcription factors, which regulate lipid metabolism and inflammation. Murine J774.2 macrophages were stimulated with Escherichia coli lipopolysaccharide (concentration, 10 mu g/mL) with or without the PPAR gamma ligand, 15-deoxy-Delta (12,14) prostaglandin J(2) (15d-PGJ(2)), or the LXR alpha ligands, 22(R)-hydroxycholesterol and T0901317 (concentration range, 0.01-10 mu mol/L), alone or in combination. Nitric oxide (NO) metabolites and tumor necrosis factor a production, inducible NO synthase expression, and mitochondrial respiration were measured. When added to the cells as single agents, 15d-PGJ(2), 22(R)-hydroxycholesterol, or T0901317 reduced the lipopolysaccharide-induced NO and tumor necrosis factor a production and the inducible NO synthase expression, and partially maintained mitochondrial respiration in a concentration-dependent manner. When added to the cells in combination at suboptimal concentrations, 15d-PGJ2 with 22(R)-hydroxycholesterol, or 15d-PGJ2 With T0901317, exerted anti-inflammatory effects similar to much higher concentrations (10,000-fold to 100,000-fold) of each ligand alone. The anti-inflammatory effects of these ligands, alone or in combination, were associated with reduction of nuclear factor-kappa B activation and with enhancement of PPAR gamma DNA binding. LXR alpha expression was upregulated in response to 15d-PGJ(2) and to the LXR alpha ligands when added alone or in combination. Immunoprecipitation experiments revealed that PPAR gamma interacted with LXR alpha. Our data demonstrate that the PPAR gamma ligand, 15d-PGJ(2), and the LXR alpha ligands, 22(R)-hydroxycholesterol and T0901317, although binding to different nuclear receptors (i.e., PPAR gamma and LXR alpha, respectively), affect mediator production through common cell signaling events and exert a synergistic potentiation in a combined treatment at suboptimal concentrations. Thus, our data suggest that PPAR gamma and LXR alpha may interact in controlling the inflammatory response in macrophages.